Cardiac stimulating device

ABSTRACT

An implantable cardiac stimulating device has a control circuit which varies the rate of stimulation pulses up to a maximum pacing rate. A sensor senses at least one evoked response parameter to a delivered stimulation pulse, and the control circuit compares a time gap between the stimulation pulse and its associated evoked response parameter. The control circuit lowers the maximum pacing rate if the time gap does not increase as the pulse rate is increased.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an implantable cardiac stimulatingdevice of the type having a housing, a control circuit enclosed in thehousing connected to a first electrode to be positioned to stimulate afirst ventricle of the heart, the control circuit delivering stimulatingpulses to the first electrode and varying the rate of stimulating pulsesup to a maximum pacing rate, and the device having a sense amplifier forsensing at least one evoked response parameter from the first ventricle.

2. Description of the Prior Art

Cardiac stimulating devices of the above type are known in the art. Therate of stimulating pulses may be varied either in response to thesensing of an intrinsic atrial depolarization or by detecting the body'sneed for cardiac output by means of a sensor (a so-called sensorcontrolled/rate-responsive pacemaker).

Most pacers are arranged to stimulate the right ventricle of the heart,but it is also known to stimulate the left ventricle. In particular forthe treatment of congestive heart failure or other severe cardiacfailures it is known to stimulate the left ventricle, or bothventricles, in order to optimize the hemodynamic performance of theheart.

U.S. Pat. No. 5,728,40 describes a method and an apparatus for pacingthe left ventricle of the heart. The pacing electrode is positionedwithin the interventricular septum proximate the left ventricular wallthereof.

U.S. Pat. No. 5,720,768 describes different possible electricalpositions in order to stimulate or sense the different chambers of theheart.

Also the article “A Method for Permanent Transvenous Left VentricularPacing” by Blanc et al, PACE, Vol. 21, 1998, pp. 2021-2024, describes amethod for positioning leads for left ventricular pacing.

U.S. Pat. No. 4,928,688 describes a method and an apparatus for treatingpatients suffering from congestive heart failure by stimulating both theventricles. The document discusses the problem of the left and rightventricles contracting asynchronously. In order to effect substantiallysimultaneous contraction of both ventricles, the document suggests meansfor separately processing sensed cardiac signals from each of the rightand left ventricles. If ventricular contractions are not sensed in boththe ventricles within a period of coincidence defined by a time delay,the pacing pulse will be emitted at the end of this time delay, but onlyto the ventricle for which a QRS-complex has not been sensed. The timedelay is suggested to be in the order of 5-10 ms.

SUMMARY OF THE INVENTION

Pacemakers are becoming more and more automatic in their functions. Onesuch automatic function is that the pacemaker includes means for varyingthe rate of stimulating pulses, i.e. the pacing rate. Thereby, thepacemaker normally has a preset maximum pacing rate. From literature itis known that a progressive heart disease may alter the compliancepatterns due to geometric remodeling of the myocardium. Such aremodeling may lead to different problems, it may for example result ina desynchronization of the ventricles, in particular at higher pacingrates.

The present invention is based on the recognition that the time gapbetween a stimulating pulse and the associated evoked response parametermay be monitored in order to detect heart problems, such asdesynchronization. Normally, when the pacing rate increases, the timegap between a stimulating pulse and the associated evoked responseparameter becomes shorter. However, at a certain pacing rate, this timegap may stop decreasing although the pacing rate increases. The presentinvention is based on the recognition that such a situation is anindication of heart problems, such as a desynchronization between theventricles. It is an object of the present invention to provide animplantable cardiac stimulating device wherein safety for the patientcarrying the device will be increased, should any impendingdesynchronization situation as described above occur.

The above object is achieved in accordance with the invention in animplantable cardiac stimulating device having a housing containing acontrol circuit which is connectible to a first electrode that ispositionable to stimulate a first ventricle of the heart, the controlcircuit including a pulse generator for supplying stimulating pulses tothe first electrode, a rate varying circuit for varying the rate of thestimulating pulses up to a maximum pacing rate, and an evoked responsesensor for sensing at least one evoked response parameter of the firstventricle to the delivered stimulating pulses, and wherein the controlcircuit includes a timer which measures a time gap between a stimulatingpulse and the associated evoked response parameter sensed by the evokedresponse sensor, a monitor circuit which monitors the time gap for thevarying pacing rates at which the stimulating pulses are delivered, andwherein the control circuit lowers the maximum pacing rate if the timegap does not decrease with an increasing pacing rate.

Since, according to the invention, the maximum pacing rate is lowered ifthe aforementioned time gap does not decrease with increasing pacingrate, the risks to which the patient is exposed are reduced.

In a further embodiment of the invention, the control circuit stores themeasured time gap for one or more pacing rates, and compares thecurrently measured time gap with a previously stored time gap for thecorresponding pacing rate, and lowers the maximum pacing rate also ifthe difference between the currently measured time gap and thecorresponding stored time gap exceeds a predetermined value. In thisembodiment a comparison thus is made between a current time gap and acorresponding stored time gap. The stored time gap may be, for example,a time gap measured one or more days before the present time gap ismeasured. The stored time gap may represent a normal time gap for thepatient in question. The fact that the present time gap exceeds thestored time gap with a predetermined value is an indication of a heartproblem; a previous heart problem may, for example, have become worse.In response to the detected problem, the maximum pacing rate thus islowered in order not to expose the patient to high risks.

In a further embodiment of the invention, the control circuit monitorsthe change in time gap ΔG when the pacing rate increases, and thecontrol circuit lowers the maximum pacing rate if the change in time gapΔG is below a predetermined value. In this embodiment the maximum pacingrate may be lowered before the aforementioned time gap starts decreasingwith increasing pacing rate.

In another embodiment of the invention, the maximum pacing rate includesat least one of the maximum sensor rate and the maximum track rate. Themaximum sensor rate is a programmable value in rate-modulated pacingsystems. When the sensor is controlling the pacing rate, the pacing ratewill not exceed the programmed maximum sensor rate. The maximum trackingrate is a programmable value in dual-chamber sensing and tracking modes.The maximum tracking rate determines the highest ventricular pacing ratethat can be achieved in response to atrial sensed events. The maximumtracking rate is also called the ventricular tracking limit at thehighest synchronous rate.

In a further embodiment of the invention, the control circuit isconnected to a second electrode which is positionable to stimulate asecond ventricle of the heart, and includes a pulse generator forsupplying stimulating pulses to the second electrode, and another evokedresponse sensor for sensing at least one evoked response parameter tothe stimulation of the second ventricle. The aforementioned timer inthis embodiment also measures a second time gap between a stimulatingpulse delivered to the second ventricle and the associated evokedresponse parameter of the second ventricle. The control circuit lowersthe maximum pacing rate if at least one of the aforementioned (first)time gap or the second time gap does not decrease with increasing pacingrate. This embodiment allows for bi-ventricular pacing.

In a further embodiment of the invention, the control circuit lowers themaximum pacing rate also if the difference between the first and secondtime gaps exceeds a predetermined value. If the difference between thefirst and second time gaps is too large, this is also an indication thatthe heart does not respond properly to the pacing. In order to reducethe risks to which the patient is exposed, the maximum pacing rate isthus lowered.

In a further embodiment of the invention, the control circuit includesan enable unit which enables the delivery of the stimulating pulses tothe first and second electrodes within the same cycle of the heart, witha time interval therebetween, and the control circuit varies the timeinterval. The control circuit in this embodiment identifies a first timeduration from the time of delivery of a stimulation pulse to the firstventricle to the sensing of an associated evoked response parameterthereto, and a second time duration from the time of delivery of astimulation pulse to the second ventricle to the sensing of an evokedresponse parameter thereto. The control circuit includes a comparatorfor comparing the first and second time durations, and the controlcircuit controls delivery of the stimulating pulses to the first andsecond electrodes to minimize any difference between the first andsecond durations. Since in this embodiment the stimulating pulses aredelivered to the ventricle with a time interval therebetween, it ispossible to synchronize the sensed evoked response parameters for theleft and right ventricles. Such synchronization is important withpatients with severe congestive heart failure.

In a further embodiment of the invention, the sensor for sensing atleast one evoked response parameter to stimulation for the first and/orsecond ventricles senses an electrical evoked response parameter. Suchan electrical evoked response parameter may be sensed by, for example,the electrode or electrodes used for stimulating the ventricles.

In another embodiment of the invention, the sensor for sensing at leastone evoked response parameter to stimulation for the first and/or secondventricle senses a mechanical evoked response parameter. The mechanicalevoked response parameter may constitute the actual contraction of theventricle or ventricles. Such a mechanical response parameter may besensed by, for example, an accelerometer, a pressure sensor or animpedance sensor. An advantage with the sensing of a mechanical evokedresponse parameter is that this parameter is directly indicative of thecontraction of the ventricles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a device according to theinvention connected to a heart.

FIG. 2 is a block diagram of a device according to the invention.

FIG. 3 schematically illustrates an electrocardiographic response signalto a stimulating pulse.

FIG. 4 schematically illustrates the relationship between the time gapand the pacing rate.

FIGS. 5 a, 5 b and 5 c respectively schematically illustrate typicalelectrocardiographic response signals to stimulation of the left andright ventricles.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an implantable cardiac stimulating device 10, hereinafteralso called a pacemaker, according to the invention. The pacemaker 10has a housing 12. A control circuit 14 (see FIG. 2) is enclosed in thehousing 12. The control circuit 14, and thereby the pacemaker 10, isadapted to be connected to a first electrode 16. FIG. 1 shows such anelectrode 16 which is connected to the pacemaker 10 via a lead 13. Thefirst electrode 16 is adapted to be positioned to stimulate a firstventricle 18 of the heart 19. The first ventricle 18 is in this case theright ventricle. According to an embodiment of the invention, thepacemaker 10 is adapted to be connected to a second electrode 36. FIG. 1shows such a second electrode 36 connected to the housing 12 via a lead15. The second electrode 36 is positioned to stimulate a secondventricle 38 of the heart 19. The second ventricle 38 is in this casethe left ventricle. The electrodes 16, 36 may include more than oneelectrical conductor in order to allow for bipolar pacing and sensing.

FIG. 2 is a block diagram of a control circuit 14 which is enclosed inthe housing 12 of the pacemaker 10. The control circuit 14 includes apulse generator 20 for delivering stimulating pulses 21 to the firstelectrode 16. The control circuit also includes a rate varying unit 22for varying the rate of stimulating pulses up to a maximum pacing rateM. The maximum pacing rate M may be the maximum sensor rate and/or themaximum track rate. The control circuit 14 also includes a senseamplifier 24 for sensing at least one evoked response parameter of saidfirst ventricle 18 to the stimulating pulses delivered via the firstelectrode 16. The evoked response parameter may either be a parameterwhich indicates a mechanical contraction of the ventricle 18 or aparameter indicating an electrical response. The mechanical contractionmay, for example, be sensed by an accelerometer, a pressure sensor or animpedance sensor. The impedance may, for example be sensed by anelectrode 16, 36 connected to the pacemaker 10. The evoked responseparameter may also be an electrical evoked response parameter which issensed, for example, by the electrode 16, 36 positioned in theventricle. Such an electrical evoked response parameter may be, forexample, the T-wave or the R-wave in the electrical evoked response.

The control circuit 14 also has a time measurement unit 26 to measure afirst time gap G between a stimulating pulse and the associated evokedresponse parameter sensed by the sense amplifier.

FIG. 3 shows a schematic representation of a typical electrical sensedresponse signal. A stimulating pulse is represented by the referencenumber 21. In the electrical response to such a stimulating pulse 21 anR-wave (also called QRS-complex) and a T-wave may be detected. In theexample according to FIG. 3 the sensed evoked response parameter is theT-wave. G represents the aforementioned time gap between the stimulatingpulse 21 and the associated evoked response parameter sensed by thesense amplifier 24.

FIG. 2 also shows that the control circuit 14 includes a monitor 28 formonitoring the first time gap G at the varying pacing rates with whichthe stimulating pulses 21 are delivered. The control circuit 14 lowersthe maximum pacing rate M if the first time gap G does not decrease withincreasing pacing rate.

FIG. 4 shows a schematic representation of the relationship between thetime gap G and the pacing rate. The pacemaker 10 normally has a preset,programmable maximum pacing rate M. The maximum pacing rate isrepresented with M in FIG. 4. The time gap between a stimulating pulse21 and the associated evoked response parameter normally decreases whenthe pacing rate increases. However, for some patients, for example thosewith a progressive heart disease which may alter the compliance patternsdue to geometric remodeling of the myocardium, the heart disease may besuch that the previously set maximum pacing rate M is in fact too highfor the patient. According to the present invention, the maximum pacingrate M is lowered if the mentioned first time gap G does not decreasewith increasing pacing rate. In FIG. 4 the point 29 on the curve is apoint where the mentioned time gap G does not decrease with increasingpacing rate. When this point 29 is reached, the maximum pacing rate M isthus lowered according to the present invention.

In a preferred embodiment of the invention the control circuit 14includes a monitor for monitoring the change in time gap ΔG when thepacing rate increases. The control circuit 14 lowers the maximum pacingrate M if the change in time gap ΔG is below a predetermined value. InFIG. 4 two examples of ΔG are indicated. ΔG₁ is relatively large and ΔG₂is smaller. When ΔG is below a predetermined value the maximum pacingrate M is thus lowered. Thereby the maximum pacing rate M may be loweredbefore the point 29 is reached. The risks to which the heart is exposedthus are reduced even further.

Returning to FIG. 2, the control circuit 14 also has a memory 30 forstoring the measured first time gap G for one or more pacing rates. Thecontrol circuit 14 further includes a comparator 32 for comparing thepresent measured first time gap with a previously stored first time gapfor the corresponding pacing rate. The control circuit 14 lowers themaximum pacing rate M also in case of difference between the presentmeasured first time gap and the corresponding stored first time gapexceeds a predetermined value. Thereby a further measure is made inorder to reduce the risk for the patient.

As explained above in connection with FIG. 1, the pacemaker 10 may beadapted to be connected to a second electrode 36. In this embodiment,the control circuit 14 includes a pulse generator 40 (see FIG. 2) fordelivering stimulating pulses also to the second electrode 36. Thepacemaker 10 also includes a sense amplifier 44 for sensing at least oneevoked response parameter to the stimulation of the second ventricle 38.The unit 26 which measures the first time gap G is also arranged tomeasure a corresponding second time gap between a stimulating pulse andthe associated evoked response parameter of the second ventricle 38. Thecontrol circuit 14 lowers the maximum pacing rate M if at least one ofthe first and second time gaps does not decrease with increasing pacingrate. This embodiment thus has the advantage that both ventricles aremonitored.

In still another embodiment, the control circuit 14 lowers the maximumpacing rate M also if the difference between the first and second timegaps exceeds a predetermined value. A relatively large difference intime gap G between the left and right ventricles is an indication thatthe heart does not respond properly to the pacing. Thus the maximumpacing rate M is lowered also in this case, in order to reduce the risksto which the patient is exposed.

According to the embodiment shown in FIG. 2, the control circuit 14 alsoincludes a pulse generator 46 arranged to enable the delivery of thestimulating pulses to the first 16 and the second 36 electrodes withinthe same cycle of the heart such that there may be a time interval dTbetween them. Furthermore, the control circuit 14 varies the timeinterval dT. The control circuit 14 has a comparator 48 that comparesthe occurrence in time of the sensed evoked response parameter to thestimulation of the first ventricle 18 with the sensed evoked responseparameter to the stimulation of the second ventricle 38. The controlcircuit 14 also includes a pulse delivery controller 50 to control thedelivery of the stimulating pulses to the first electrode 16 and theelectrode 36 such that the difference in occurrence in time ΔT betweenthe sensed evoked response parameter to the stimulation of the firstventricle 18 and the sensed evoked response parameter to the stimulationof the second ventricle 38 is minimized. This embodiment is furtherillustrated in FIGS. 5 a, 5 b, 5 c. The evoked response parameter may bea mechanical or an electrical sensed evoked response parameter asexplained above. FIGS. 5 a, 5 b, 5 c show typical electrical evokedresponses. The electrical evoked response parameter may be related toeither the R-wave or the T-wave. Moreover, different alternatives existfor detecting the evoked response. The evoked response parameter may forexample be a peak or a maximum 52 or a certain predetermined slope 54,55 of the wave which is detected. Also other possible points on thecurve in the electrical evoked response may be detected, e.g. azero-crossing. Instead of directly comparing the occurrence in time of aslope or peak or other point on the respective wave, it is possible tomeasure an integral of the difference between the wave in question inthe evoked response to the stimulation of the first ventricle 18 and thewave in the evoked response to the stimulation of the second ventricle38. The time interval dT is thereby set such that the integral isminimized. The sensing of the evoked response may be done either with aunipolar or with a bipolar arrangement. When the R-wave is sensed it maybe advantageous to use a unipolar sensing. When the T-wave is sensed itmay be advantageous to use bipolar sensing.

In FIG. 5 b an example is shown where the peak of the R-wave isdetected. The curve 56 represents the electrical evoked response to astimulation pulse 21 for the first ventricle 18. The curve 57 representsthe corresponding evoked response for the second ventricle 38. Accordingto this example, the stimulating pulses 21 to the first electrode 16 andthe second electrode 36 are delivered simultaneously. In the exampleshown, the peak of the curve 56 occurs before the peak of the curve 57.The difference in occurrence in time between the peaks of the curves 56and 57 is represented by ΔT. In this embodiment of the invention, thecontrol circuit 14 has a pulse generator thus 50 which delivers thestimulating pulses to the first electrode 16 and the second electrode 36at different times such that the difference in occurrence in time ΔT ofthe peaks of the curves 56 and 57 is minimized.

FIG. 5 c illustrates that the stimulating pulse to the second ventricle58 is delivered before the stimulating pulse to the first ventricle 18.The two peaks of the curves 56 and 57 occur substantiallysimultaneously. In order to make the peaks occur simultaneously it ispossible to either deliver the stimulating pulse to the electrode 36(corresponding to the curve 57) earlier in time or to deliver the pulseto the electrode 16 (corresponding to the curve 56) later in time. Aphysician may determine which of the two possibilities is most suitablefor a particular patient.

Although modifications and changes may be suggested by those skilled inthe art, it is the intention of the inventors to embody within thepatent warranted hereon all changes and modifications as reasonably andproperly come within the scope of their contribution to the art.

1. An implantable cardiac stimulating device comprising: a housing; acontrol circuit contained in said housing; an electrode adapted forpositioning to stimulate a ventricle of a heart; a pulse generatorconnected to said control circuit and to said electrode for generatingstimulation pulses at a pulse rate; a pulse rate variation unitconnected to said pulse generator for varying said pulse rate up to amaximum rate; an evoked response sensor connected to said electrode forsensing an evoked response parameter of said ventricle to a stimulationpulse delivered to said first ventricle via said electrode; a timemeasurement unit connected to said sense amplifier for measuring a timegap between one of said stimulating pulses and an associated evokedresponse parameter sensed by said evoked response sensor; a monitorconnected to said time measurement unit for monitoring said time gap ata plurality of different pulse rates varied by said pulse rate variationunit; and said control circuit lowering said maximum rate if said timegap does not decrease as said pulse rate increases.
 2. An implantablecardiac stimulating device as claimed in claim 1 further comprising amemory for storing said time gap for at least one of said pacing rates,as a stored time gap, and a comparator for comparing said stored timegap with a current time gap, and wherein said control circuit lowerssaid maximum pacing rate if a difference between said current time gapand said stored time gap exceeds a predetermined value.
 3. Animplantable cardiac stimulating device as claimed in claim 1 whereinsaid control circuit includes a change monitor for monitoring a changein said time gap as said pulse rate increases, and wherein said controlcircuit lowers said maximum rate if said change in said time gap isbelow a predetermined value.
 4. An implantable cardiac stimulatingdevice as claimed in claim 1 wherein said maximum rate is selected fromthe group consisting of a maximum sensor rate and a maximum track rate.5. An implantable cardiac stimulating device as claimed in claim 1wherein said electrode is a first electrode, said ventricle is a firstventricle, said stimulation pulse generator is a first stimulation pulsegenerator, said evoked response sensor is a first evoked response sensorand wherein said time gap is a first time gap, said implantable cardiacstimulating device further comprising: a second electrode adapted to bepositioned in a second ventricle of the heart; a second stimulationpulse generator connected to said second electrode for generatingstimulation pulses at said pulse rate for delivery to said secondventricle via said second electrode; a second evoked response sensorconnected to said second electrode for sensing an evoked responseparameter of said second ventricle to said stimulation pulses deliveredvia said second electrode; and said time measurement circuit alsomeasuring a second time gap between a stimulation pulse supplied viasaid second electrode and an associated evoked response parameter ofsaid second ventricle sensed by said second evoked response sensor, andsaid control circuit lowering said maximum rate if at least one of saidfirst time gap and said second time gap does not decrease as said pulserate increases.
 6. An implantable cardiac stimulating device as claimedin claim 5 further comprising a difference former for forming adifference between said first time gap and said second time gap, andwherein said control circuit lowers said maximum rate if said differenceexceeds a predetermined value.
 7. An implantable cardiac stimulatingdevice as claimed in claim 5 further comprising: a pulse deliverycontroller connected to said first and second stimulation pulsegenerators for controlling delivery of said stimulation pulsesrespectively via said first electrode and said second electrode with atime interval therebetween; a comparator for comparing a time ofoccurrence of the evoked response parameter sensed by said first evokedresponse sensor to the time of occurrence of the evoked responseparameter sensed by said second evoked response sensor to identify atime difference therebetween; and said pulse delivery controllercontrolling delivery of said stimulation pulses by said first and secondpulse generators so that said time difference is minimized.
 8. Animplantable cardiac stimulating device as claimed in claim 5 whereinsaid first evoked response sensor is a sensor for sensing an electricalevoked response parameter of said first ventricle and wherein saidsecond evoked response sensor is a sensor for sensing an electricalevoked response parameter of said second ventricle.
 9. An implantablecardiac stimulating device as claimed in claim 5 wherein said firstevoked response sensor is a sensor for sensing a mechanical evokedresponse parameter of said first ventricle and wherein said secondevoked response sensor is a sensor for sensing a mechanical evokedresponse parameter of said second ventricle.
 10. An implantable cardiacstimulating device as claimed in claim 1 wherein said evoked responsesensor is a sensor for sensing an electrical evoked response parameterfrom said ventricle.
 11. An implantable cardiac stimulating device asclaimed in claim 1 wherein said evoked response sensor is a sensor forsensing an mechanical evoked response parameter from said ventricle.